A novel self-Q-switched and orthogonally polarized dual-wavelength laser has been investigated with Yb3+-doped CGB disordered crystals. By slightly inclining output coupler to introduce the Fresnel loss, we realized simultaneously dual-wavelength laser operation at 1052.6 nm in E//b polarization and 1057.7 nm in E//c polarization with a frequency difference of 1.38 THz. Self-Q-switched pulse generation was observed in this free-running laser, originating from the nonlinear reabsorption effect of Yb:CGB as well as the strong storage of inversion population induced by the long excited-state lifetime (~1 ms). Pulse duration of 287 ns was obtained with an output average power of 416 mW and a repetition rate of 35 kHz. The self-Q-switching effect increased the peak power by 100 times the average power. This very simple laser, free from the complexity and high-cost of additional intracavity polarization modulator and Q-switcher, may be applied for constructing miniature, integrated and portable laser system.
© 2014 Optical Society of America
In the last years, intensive research has been carried out on the orthogonal-polarization dual-wavelength lasers because of their significant applications in self-sensing metrology, medicine, holography, and precision spectroscopy [1–4]. In particular, frequency mixing of the dual-wavelength laser through nonlinear crystal or photoconductive antenna is an available and convenient method for efficient terahertz radiation [5,6]. Such orthogonally polarized dual-wavelength lasers have been successfully demonstrated with a variety of Nd3+ doped crystals such as Nd:GdVO4, Nd:LuVO4, Nd:YAG, and Nd:YLF by utilizing additional intracavity birefringent element, etalon, and the joint of two cavities [6–9]. In comparison with Nd3+ crystals, thanks to the simple two-level electronic structure of Yb3+ ions, Yb3+ counterparts have the well-known merits as no excited-state absorption, low quantum defect (generally less than 10%), and weak heat loading, which are definitely advantageous to improve laser efficiency and reduce thermal-induced frequency and power jitter [10,11]. This has been proved by the active studies on the orthogonally polarized dual-wavelength Yb:GAB and Yb:KGW lasers, realized with separately pumping each polarized beam and by inserting chirped volume Bragg gratings into the cavity, respectively [12,13]. However, these Nd and Yb lasers suffer from the undesirable mechanical instability, complexity, high cost, and insertion loss of the additional optical elements. This is why the development for a compact orthogonal-polarization dual-wavelength laser without any additional element is of high interest. Recently, a good designing idea for such a laser has been given in Ref , where dual-wavelength operation was achieved in a Yb:GAB laser by translating the cavity mirror.
As Yb3+ ion belongs to the typical quasi-three-level system, its energy-level distribution at the ground state 2F7/2 and excited state 2F5/2 is highly sensitive to the crystal field, exhibiting a strong dependence of spectroscopic and laser characteristics on the host medium. For instance, strong crystal field generates strong Stark splitting, and thus results in a broad gain band [15,16]. In view of this, to realize a dual-wavelength laser with frequency difference large enough for promising terahertz wave generation, host candidate with strong crystal field is desirable. A novel double borate laser crystal, namely Yb3+:Ca3Gd2(BO3)4 (Yb:CGB), was firstly reported by P.-H. Haumesser et al. [16,17]. Its laser potential has been evaluated in our recent works [18,19]. In its orthorhombic crystallographic structure, the Yb3+, Ca2+ and Gd3+ cations occupy the lattices of B3+ sets statistically. This leads to a high degree of structural disorder and strong crystal field, exhibiting good capacity for simultaneously multi-wavelength laser generation . Another favorable property of Yb:CGB is that the inside laser emission is naturally orthogonal-polarization, which originates from the optical anisotropy and birefringence of the asymmetric crystal structure . So Yb:CGB should be a suitable candidate for constructing dual-polarization and dual-wavelength lasers.
For many practical applications, laser source with relatively high peak power is favorable. Further increasing the peak power can be realized by Q-switching technique, which traditionally requires saturable absorber or active modulator as Q-switcher. The structural complexity and insertion loss of Q-switchers inevitably trouble the laser design. We now introduce a simple and reliable self-Q-switched orthogonal-polarization dual-wavelength Yb:CGB laser by appropriately adjusting the loss induced from the Fresnel reflection on the output coupler (OC). The difference of the two emission wavelengths lied inside THz region. The combination of the nonlinear reabsorption effect and strong inversion population accumulation of the long excited-state lifetime led to a self-Q-switching operation that increased the peak power up to 100 times the average output power. Comparing to traditional Q-passive and active Q-switchers, the self-Q-switching effect reduces the laser volume and cost considerably. This low-cost Yb:CGB laser with high polarization at two wavelengths may meet the requirements of those aforementioned applications.
2 Resonator design and experiment setup
For purpose of comparison, we grew Yb:CGB crystals with three Yb3+-doping concentrations (5, 10, and 15 at.%) by the Czochralski method. The excited-state lifetimes were measured to be up to 0.83, 0.97, and 1.14 ms for 5%, 10%, and 15% Yb concentrations, respectively, much longer than many known Yb3+ crystals. The crystallographic axes (a, b and c) are perpendicular to each other in the orthorhombic structure. The three crystals were a-cut with a length of 5 mm and a cross section of 3 × 3 mm2. The gain cross sections for E//b and E//c electric field polarizations in 1.0-1.1 μm region with a common population inversion rate of 0.5 are shown in Fig. 1. It reveals two gain competitions existing in lasing operation: one is between the two polarizations, and the other is between different wavelengths. With a close observation, one can find an intersection at 1.05 μm between the gain curves of E//b and E//c for each of the three doping concentrations. Around the intersection the ratios of the gain cross sections for the two polarizations are different. That is to say, if the laser operates in the region before the intersection, it is naturally at E//b polarization as superior to E//c in gain competition, while after the intersection the situation is reversal. As a consequence, the laser polarization state is very sensitive to the cavity loss due to gain competition. Even a little difference between the losses of the two polarizations will change the gain competition since their gain cross sections are so approximate. Furthermore, the emission wavelength of a laser with wide gain band is strongly influenced by the balance of gain and loss. In our previous study, the Yb:CGB crystals exhibited wideband absorption ranging from ~850 to ~1060 nm with peak absorption at 976 nm . It is found that the laser emission of the Yb:CGB crystal mainly centered at 1.05 μm rather than the shorter wavelength (near the absorption peak) with higher gain . That is a result of the reabsorption loss existing generally in the quasi-three-level configuration. The Yb:CGB laser also has the capacity for simultaneously dual-wavelength laser generation under relatively low pump intensity which is insufficient to support the oscillation of more frequencies within the gain band . This gives us an opportunity to achieve an orthogonally polarized dual-wavelength laser operation. If the cavity losses of the two competing wavelengths at different sides of the gain intersection can be moderately adjusted to realize gain-to-loss balance in both the polarization and wavelength, an orthogonally polarized dual-wavelength laser will be done.
In order to compact the laser structure, our purpose is to adjust cavity loss by a simple linear cavity only including laser medium and cavity mirrors, as shown in Fig. 2 schematically. The pump source was a fiber coupled, unpolarized continuous-wave diode laser at 976 nm with a core diameter of 400 μm and a numerical aperture of 0.22. The three uncoated Yb:CGB samples were wrapped by indium foil and mounted in a copper block kept at 24 °C to remove the stored heat. With a 1:1 collimation and focus system, the pump laser was refocused into the crystal. The plane input mirror (IM) was antireflection coated at 976 nm and high-reflection coated at 1010-1070 nm. Two concave spherical OCs, with the curvature radius of 75 mm and transmittances of 3% and 5% at 1010-1070 nm, were utilized to compare the influence on the laser performance. The cavity length was compressed to 7 mm. Considering the thermal lens effect of laser crystal is crucial for cavity design, we measured the relationship between the focal length of the thermal lens and pump level [see Fig. 3(a)]. The focal length decreases monotonously as the incident pump power increased. For this plane-concave geometry, when the thermal lens effect of Yb:CGB is taken into account, the half-divergence angle of the oscillation laser beam is less than 0.21 degree, mainly because of the short cavity length and relatively large curvature radius of the OC. Therefore, the incidence of the oscillation laser on the OC can be approximate to normal incidence on a plane interface. Under this condition, the losses for S (perpendicular to the plane of incidence) and P (parallel to the plane of incidence) waves induced by the Fresnel reflection of the OC can be expressed as [8,20]Fig. 3(b), the tendency of the Fresnel loss for S polarization with increasing the inclined angle is obviously opposite to that for P polarization even if the angle is only within 0.6 degree. Meanwhile, the loss for short wavelength is higher than that for long one due to the higher refractive index. That certainly allows us to adjust the loss for different polarizations and wavelengths by inclining the OC to realize gain-to-loss balance in both the polarizations and wavelengths, without serious cavity misalignment. In this case, the Yb:CGB samples were placed with the laser propagation direction along a axis, b axis parallel to the horizontal axis, and c axis parallel to the vertical axis. The inclination of the OC was adjusted in the vertical direction, so that the vertical plane is the plane of incidence as E//b and E//c polarizations refer to S and P waves, respectively. The OCs were placed into a high-stability optical mount that we could accurately adjusted their inclination relative to the cavity axis. The polarization of the output laser was distinguished by a polarization beam splitter. The spectrum analyzer used in this experiment has a resolution of 0.02 nm. A He-Ne laser was guided to the rear surface of the OC that the OC inclined angle could be calculated from the offset of the reflected light.
3. Experimental results and discussions
The simultaneous dual-wavelength laser around 1.05 μm operated at separate polarization was achieved in all the six cases (the six combinations of the three Yb:CGB samples and two OCs) by varying θ in the region of 0.19-0.31 degree. As indicated in Fig. 3(b), the θ regions are slightly different for different combinations. The total output power, recorded when the power ratio of the two polarizations was adjusted to 1:1, is dotted in Fig. 4(a). Increasing the pump power exceeding these scales in Fig. 4(a) led to strong gain competition and turned the dual-wavelength laser to multi-wavelength emission. The output power of the 5% Yb:CGB was lower than that of the 10% and 15% ones for the lower gain cross section. But the lower power of the 15% Yb:CGB compared with the 10% counterpart should due to the internal defects, such as dislocation and inclusions, caused by high doping concentration. The emission wavelengths were centered at 1050.9 (E//b) and 1056.9 (E//c) nm, 1052.6 (E//b) and 1057.7 (E//c) nm, as well as 1053.4 (E//b) and 1059.2 (E//c) nm for the 5%, 10%, and 15% Yb:CGB lasers, respectively, with the frequency differences of 1.62, 1.38, and 1.60 THz. The maximum output power of 713 mW was obtained under 3.55 W of pump power with the 10% Yb:CGB and 3% OC. The corresponding spectrum with double emission peaks is depicted in Fig. 4(b). One can see that the two wavelengths had the same emission intensity, which is most beneficial for practical applications.
In the six cases, three laser operation regimes, i.e. E//b linear polarization at single wavelength, orthogonal polarization separately at two wavelengths, and E//c linear polarization at single wavelength, were all observed during the adjustment of θ. Under the pump power of 3.55 W with the 10% Yb:CGB and 3% OC, for instance, the laser operated at 1052.6 nm along E//b polarization for θ less than ~0.19 degree [see the left inset of Fig. 4(b)], and just then the output power increased to 785 mW; whereas the laser turned to 1057.7 nm at E//c polarization instead of 1052.6 nm as θ was large than ~0.22 degree [see the right inset of Fig. 4(b)], and the output power decreased to 323 mW. The simultaneously dual-wavelength laser was achieved between the two single-wavelength regimes.
The other important feature of this Yb:CGB laser is the spontaneous output of Q-switched pulses. The self-Q-switching was insensitive to the operation wavelength but strongly depended on the pump intensity. By using an oscilloscope with 4 GSa/s sampling rate and 500 MHz electrical bandwidth, we observed self-Q-switching operation in all the six cases in low-pump regions as indicated by arrows in Fig. 4(a). When the pump power was increased outside these regions, the pulse waveform trended to disorder, and even at relatively high pump power it degenerated to continuous wave. The measured pulse duration and repetition rate in these self-Q-switching regions are shown in Figs. 5(a) and 5(b), respectively. As increasing the pump power, the duration and repetition rate exhibit monotonous drop and rise, respectively, indicating the self-Q-switching behavior to be similar as a common passively Q-switched laser with saturable absorber. By use of the 10% Yb:CGB and 3% OC, the minimum pulse width of 287 ns with a pulse repetition rate of 35 kHz and an average output power of 416 mW was obtained under the pump power of 2.9 W, giving a pulse energy of 11.9 μJ. The self Q-switching dynamics produced a peak power of 41 W, nearly 100 times the average output power. But it is also should be noted that the pulse amplitude fluctuation was measured to be ~47%, with pulse timing jitters being ~11 ns and pulse duration fluctuation being ~54%, showing a large potential for further optimization. Figure 6 shows the self-Q-switched pulse trains detected in 450 μs span, demonstrating good amplitude stability. The pulse had a regular Gauss-like profile, as shown in left inset of Fig. 6. The corresponding spectrum has two equal-intensity peaks at 1052.6 and 1057.7 nm, very similar to Fig. 4(b). As the spatial profile presented in the right insert of Fig. 6, the beam energy was highly concentrated, suggesting that the laser operated on single transverse mode. The pulse waveform in Fig. 6 is somewhat similar to relaxation oscillation. However, it is worth noticing that the relaxation oscillation often occurs as the pump is just above threshold where the pump has abrupt fluctuations. Thus the relaxation oscillation is an unstable regime, with the pulse width commonly above μs level. Under the threshold of this Yb:CGB laser, we only observed noise-like waveform.
During this experiment, the self-Q-switched phenomenon and dual-wavelength effect are independent. The self-Q-switch behavior was holding in both double-wavelength and single-wavelength regimes. We believe the self-Q-switching operation is caused by the following two reasons. As a quasi-three-level laser medium with the lack of excited state absorption, Yb:CGB reabsorbs the photons at the transition identical to that of laser radiation. Particularly for small gain, e.g. under relatively low pump, the loss exceeds the gain that blocks stimulated amplification and prevents laser oscillation. To characterize the reabsorption effect, transmission measurements were performed for the three Yb:CGB crystals by using an acousto-optic Q-switched laser at 1.05 μm with the pulse duration being 110 ns. As shown in Fig. 7, the transmittance was nonlinearly related to the pulse intensity. The samples became transparent to the photons as the pulse intensity increased to the maximum value. From the fit to the data, the modulation depths ΔT were extracted to be ~9.8%, ~6.7%, and ~3.9%, nonsaturable losses to be 3.9%. 6.7%, and 9.8%, saturation intensity to be 0.08, 0.07 and 0.13 MW/cm2 for the 5%, 10%, and 15% doped samples, respectively. On the other hand, Yb:CGB crystals have very long excited-state lifetime, which ensures considerable accumulation of inversion population during reabsorption. When the ground state is depleted and the excited state is sufficiently populated, the reabsorption loss decreases rapidly, causing the build of a Q-switched pulse. In view of that, short lifetime cannot offer enough inversion population accumulation to generate self-Q-switched pulse even though nonlinear reabsorption effect exists. The two mechanisms make Yb:CGB behave as a complex of gain medium and saturable absorber. So just like a common passively Q-switched laser, as the pump increases to a high level, where gain is far above the reabsorption loss, the reabsorption maintains full bleach and only low energy can be stored in the gain medium. Then, as we just mentioned, the pulses trends to erratic fluctuation and finally continuous wave. We could obtain wider pump region for self-Q-switching operation by modulating the Fresnel loss of OC to force the laser operate at shorter wavelength to improve the reabsorption effect, but orthogonally polarized dual-wavelength emission was not maintained because gain-to-loss balance was disturbed. Yb-doped medium with longer excited-state lifetime may be more promising in terms of further power an energy scaling. In addition, the high pulse repetition rates and short pulse durations we obtained should be attributed to the strong fluorescence photon intensity inside the Yb:CGB crystals, which contributes a rapid saturation recovery . This can be further explained as follows. In Yb:CGB, laser transition and reabsorption share the same excited state 2F2/5, so that the reabsorption population not only modifies intracavity loss but also participates in laser emission after reabsorption bleaching. Since the absorption cross section of Yb:CGB at the pump wavelength of 976 nm is one order higher than that at 1.05 μm , and the pump intensity is stronger than the fluorescence inside the gain medium, the accumulation of gain population (on the excited state 2F2/5) should be much rapider than that of the reabsorption population. At the moment of bleaching, both of the gain and reabsorption population will transit to ground state under the stimulation of laser photons. That is to say, the store time of reabsorption population is shorter than that of gain population. After the generation of a self-Q-switched pulse, continuous pump will soon reverses gain population and emit fluorescence spontaneously, which reboots the reabsorption for next Q-switching. As a result, the effective recovery time of reabsorption is shorter than the excited-state lifetime. And it will be further shorter with increasing the pump intensity .
In conclusion, we have realized a novel simultaneously dual-wavelength self-Q-switched Yb:CGB laser separately polarized at E//b and E//c. The laser properties of 5%, 10%, and 15% Yb doped CGB were detailedly compared. Since there is a large potential for further decreasing the resonator volume by employing a Yb:CGB microchip coupled with single 976-nm InGaAs laser diode, it is possible to construct a miniature and reliable orthogonally polarized dual-wavelength laser source for terahertz wave generation, whose peak power can be greatly increased by the self-Q-switching effect. This self-Q-switched laser may be also commensurate with the requirements of micro laser communication systems to greatly increase the signal-to-noise ratio.
This work was supported by Science and Technology Plan Major Projects of Fujian Province (2010I0015, 2012H0048), National Nature Science Foundation of China (50902129, 61078076, 91122033, 11304313), Knowledge Innovation Program of Chinese Academy of Sciences (KJCX2-EW-H03).
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